Silicon brass resistant to parting corrosion

ABSTRACT

This disclosure relates to silicon brass alloys in which corrosion resistance is obtained by the addition of a small amount of an inhibitor, such as arsenic, to provide an alloy which when properly heat treated has substantial resistance to parting corrosion in waters which contain corrosiive ingredients such as are found in many public water supplies.

United States Patent [191 Costas 1 SILICON BRASS RESISTANT TO PARTINGCORROSION [75] Inventor: Louis P. Costas, Cheshire, Conn.

[73] Assignee: The Anaconda Company, New

York, NY.

[22] Filed: Mar. 19, 1974 [21] Appl. No.: 452,618

Related U.S. Application Data [63] Continuation-impart of Ser. No.434,613, Jan. 18, 1974, abandoned, which is a continuation of Ser. No.222,508, Feb. 1, 1972, abandoned, which is a continuation-in-part ofSer. No. 887,927, Dec. 24, 1969, abandoned.

52 us. Cl. 148/32; 75/1575; 148/325; 148/115 R; 148/l2.7 [51] Int. Cl.C22c 9/10; C22f 1/08 [58] Field of Search 75/1565, 157.5, 160; 148/11.5R, 32, 32.5, 12.7

[56] References Cited UNITED STATES PATENTS 2,061,921 11/1936 Roath H75/157.5 X

1 Aug. 19, 1975 2,075,005 3/1937 Bassctt 75/1575 2,118,688 5/1938Webster..... 75/1575 2,369,813 2/1945 Wilkins.- 75/1575 2,394,673 2/1946Edmunds 75/1575 3,402,043 9/1968 Smith 75/1575 OTHER PUBLICATIONS HoracePops, "The Constitution of CopperRich Copper-Silicon-Zinc Alloys, Trans.of AIME, Vol, 230, June 1964, pp. 813-820.

Primary ExaminerC. Lovell Attorney, Agent, or Firm-Pennie & Edmonds 5 7ABSTRACT This disclosure relates to silicon brass alloys in whichcorrosion resistance is obtained by the addition of a small amoun't'ofan inhibitor, such as arsenic, to provide an alloy which when properlyheat treated has substantial resistance to parting corrosion in waterswhich contain corrosiive ingredients such as are found in many publicwater supplies.

2 Claims, 4 Drawing Figures SILICON BRASS RESISTANT TO PARTING CORROSIONRELATED APPLICATION This application is a continuation-in-part of myapplication Ser. No. 434,613 filed Jan. 18, 1974 now abandoned, whichwas a continuation of application Ser. No. 222,508 filed Feb. 1, 1972now abandoned, which in turn was a continuation-in-part of myapplication now abandoned Ser. No. 887,927 filed Dec. 24, I969.

BACKGROUND OF THE INVENTION The addition of silicon to brasses increasesthe mechanical strength of the alloys making them more suitable for useas valve stems and other objects which are subjected to tensile andbending forces. Other desirable features of these alloys aremachinability and their freedom from galling or seizing in operation.

However, the resistance of silicon brasses to the corrosive action ofmany waters and other corrosive fluids has not been satisfactory. Forexample, in some public water systems valve stems have failed within 1to 2 years due to parting corrosion. This type of corrosion can bethought of as a leaching out of alloying agents leaving copper behind ina porous and weakened condition. This process of parting corrosion isusually referred to as dezincification when it occurs in the commoncopper-zinc brasses.

The problem of dezincification in valve stems has been recognized foryears and it was particularly severe when high zinc alloys, such asmanganese bronze with 40 percent zinc, were used. To combat thisproblem, manufacturers of valves turned to the silicon brasses which cancontain from 5 to 22 percent zinc and over 0.5 percent silicon, butusually the range of 12 to percent zinc and 2.5 to 4.5 percent siliconhas been used. This approach is based on the premise that the lower zinccontent would deter dezincification and to some extent it has beensuccessful. However, the parting corrosion problem is again becomingserious because of lower water quality as a consequence of theincreasing demand for water due to increased population and industrialneeds.

As a reaction to this corrosion problem, the trend once again is towardslower zinc levels. The disadvantages of smaller zinc contents are lincreased cost of alloys and (2), what is more important, decreasing thezinc does not necessarily eliminate parting corrosion. I have discoveredthat parting corrosion in silicon brass is not solely a function of zinccontent but rather of both zinc and silicon.

Another approach has been the addition of arsenic, antimony, orphosphorus inhibitors which is known to reduce parting corrosion insingle phase copper-zinc brasses, but the use of such inhibitors hadbeen found to be ineffective for twophased copper-zinc brasses. Sincemost silicon brasses depend upon multiphase structure to produce highmechanical strength, the benefits of these inhibitors would not havebeen predicted, nor would such additions have appeared promising.Furthermore, it has not been appreciated that silicon additions aidparting corrosion of even alpha phase, the primary structure, andtherefore no prior attempts to negate the efiect of silicon have beenmade.

SUMMARY OF THE INVENTION Copper-zinc alloys containing small amounts ofsilicon in the range of 1 percent to about 2.5 percent silicon,depending on zinc content, are resistant to parting corrosion. However,when the silicon content is increased for strength purposes above theselevels to amounts in the range of greater than about 2.5% to about 7percent, depending on zinc content, parting corrosion will occur. Toovercome the corrosion disadvantage of higher silicon levels, arsenic,antimony or phosphorus will provide the alloy with substantial immunityto parting corrosion, whether the alloy is single or multiphased, ifproper heat treatment is performed on cast or wrought material.

Broadly stated, the present invention comprises a silicon brass alloywhose thermal history is controlled in such a manner that the alloyincludes substantial quantities of alpha and zeta phases and consists ofabout 3 to 20 percent zinc, about 2.5 to 6 percent silicon, from about0.030 percent up to the percentage of solid solubility in the alloy ofone or more elements of the group consisting of arsenic, antimony andphosphorus, and the remainder copper. Optionally, an amount of lead toprovide good machinability may be added to the alloy.

The principal of operation of the present invention is the selection ofproportions of copper, zinc, silicon and inhibitors within rangesdepicted by areas bordered by dashed lines on the Figures. Thedashed-lined areas disclose the alloys which under commercial conditionsof heat treatment are protectible by inhibition addition. These areas ofthe Figures show the alphazeta phase for alloys held at temperature forshort periods of time prior to quenching while the solid-line phaseboundaries show the phases, including the alpha-zeta phase as indicated,obtained when the alloys are held for long periods, such as days, atselected temperatures to permit an equilibrium condition to be reached.

The upper end of the range of inhibitor is the percentage just belowthat at which the inhibitor will start to precipitate and form compoundsthat may have deleterious effects. While percentages above about 0.10percent are not generally required, the addition of amounts ofinhibitors above 0.10 percent does not adversely affect the alloy untilthe percentage of solid solubility is passed.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1, 2 and 3 of the drawings showparting corrosion behavior for three different alloy conditions plottedon the copper-rich comer of the ternary alloy system of the presentinvention. FIG. 4 shows the phases formed when the alloy is cooled froma temperature below 500C.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1, 2 and 3 show thecopper-rich corner of the ternary system of copper, zinc and silicon andwhether an alloy having a particular composition undergoes partingcorrosion with, or without, an inhibitor present in a specific water,and under electrochemical conditions that will be described below. Theareas defined by the dashed lines denote compositions which were testedand found to suffer parting corrosion in the absence of an inhibitor.For example, in FIG. 1 it is seen that an alloy P, whose composition is8 percent zinc, 3 percent silicon, and balance copper, and cast usingnormal commercial procedures, would suffer parting corrosion. However,in accordance with my invention, the

addition of at least about 0.03 percent inhibitor pro vides substantialimmunity to parting corrosion.

The solid lines of FIGS. 2, 3 and 4 are the equilibrium phase boundariesas determined by Horace Pops (Trans. Met. Soc, AIME, 230, 813-820,1964). The significance of these diagrams is that they define the phasesthat would exist if the alloys were allowed to remain at thesetemperatures for long periods of time such as days or weeks depending onthe temperature. No equilibrium phase boundaries are shown in FIG. 1since cast alloys are generally produced under nonequilibriumconditions.

In commercial operations, the alloys of this invention are cast, thenheated and further processed by extrusion and drawing. Such treatmentsdo involve high temperatures, such as 500 to 750C, but the length oftime of treatment is generally only a few hours in duration andequilibrium is never attained. Thus, it is quite common to find, forexample, alpha and zeta phases existing in alloys where only alpha wouldbe predicted from the phase diagram. However, the phase diagram providesan excellent base on which to describe metallurgical phonomenon.

in FIGS. 2 and 3 it will be noted that the area bounded by the dashedlines involved three phases, alpha, beta, and zeta. However, beta inthis region of composition is stable only at high temperatures andattempts to retain it by very rapid quenching fall; it quicklytransforms to a mixture of alpha and zeta. On the other hand, alloysconsisting of alpha or alpha and zeta at these higher temperatures canbe quenched so that these same phases remain intact at room temperature.Thus commercial alloys quickly cooled or quenched from above about 500Ccommonly consist of alpha and zeta phases even though only alpha oralpha and beta would be predicted from the phase diagram.

Below about 500C, depending on the composition, zeta phase is no longerstable and transforms either into mu or chi as shown in FIG. 4. 1n thetemperature range of 400-500C, the kinetics of the transformation ofzeta are fast enough to produce significant amounts of mu or chi inminutes. These phases cannot be protected from parting corrosion, evenwith inhibitors present, and so they are highly undesirable from thecorrosion resistance standpoint. Alpha, of course, can be protected byinhibitor as has been known for years. The unexpected discovery thatzeta phase can also be inhibited is the basis for this invention. Thus,with proper inhibitor addition and temperature control, silicon brassalloys manufactured either in the cast or wrought form can be maderesistant to parting corrosion so long as only alpha and zeta phasesexist in the final state.

The parting corrosion test used to develop the data herein employed anelectronic potentiostat, an instrument which allows corrosion reactionsto occur under carefully defined electrochemical parameters. This testmore closely parallels actual service conditions than past proceduresusing hydrochloric acid or copper chloride solutions which are extremelyaggressive and not therefore typical of conditions found in waterdistribution systems.

The tests were carried out under an argon cover for 24 hours at zeromillivolts referenced to a saturated calomel electrode. Cylindricalspecimens were millimeters in diameter and about millimeters long. Roomtemperature test water was used, but, if no parting was detected, thetest was again run at 52C and, if still no parting was found thespecimen was categorized as showing no parting. These tests were foundto correlate well with actual experience with a variety of uninhibitedalloys which had been in service for years in aggressive potable waters.Where an alloy specimen passed both the room temperature and 52C test,it can be predicted that it would perform well in service in a corrosivewater condition. The potentiostatic tests were carried out using a testwater having similar composition and characteristics of Colorado Riverwater which is an aggressive potable water used in large quantities inthe Southwestern region of the United States. The composition of thetest water and Colorado River In compiling the corrosion data set forthon F 16. l, alloys having the ranges of zero to 24 percent zinc, l to 6percent silicon, up to about 0.06 percent of an inhibitor and with theremainder copper were cast into specimens which were then rapidly cooledto room temperature. Each specimen was thereafter tested to determine ifthe test water would cause any detectable parting corrosion. Thecriterion for determining the occurrence of parting was metallographicexamination. The following table includes the composition of a number ofspecimens tested and states whether parting corrosion was detected.

Lead was added to some of the specimens since valve stem brass usuallyincludes this element to improve machinability. lead was found to haveno effect on the corrosion properties of the alloys of the presentinvention when added in quantities as required to give goodmachinability. For example, up to about 1.5 percent lead may be added tothe alloys.

The corrosion data depicted in the ternary phase diagrams of FIGS. 2 and3 was obtained in the same manner as described in obtaining data for thediagram of FIG. 1, except that the cast specimens were swaged,encapsulated, annealed at 600C for days and 760C for 5 days and thenquenched. Compositions of some of the specimens tested and the resultsof the tests are set forth below in Table 111.

TABLE III Parting Cu 77.1 77.9 78.0 79.30 82.79 89.03 Zn 19.2 20.3 19.5217.34 13.37 6.38 Si 34 1 83 2.50 3.35 3.83 4.59 Pb 024 No Parting Cu76.96 78.79 82.58 89.07 Zn 19.66 17.24 13.47 6.28 Si 3.32 3.93 3.90 4.61Pb As 0.06 0.03 0.05 0.04

Cu 82.06 Cu 82.1] Zn 19.94 Zn 13.82 Si 3.96 Si 3.98 P 0.04 Sb 0.088

The effect of annealing specimens at 760C was to require the use of moreinhibitor at the higher zinc and silicon contents to provide immunity toparting corrosion, as illustrated by the upper dashed area on theFigures. Annealing at 600C reduced the requirement of using a largeramount of inhibitor until the zinc content reached over 19 percent.Other samples were annealed at various temperatures between 550C and760C and it was found that the immunity to parting obtained wassubstantially as illustrated on FIGS. 2 and 3. If, however, alloys areannealed or slowly cooled below about 500C, mu and chi phases will occurwhich are highly susceptible to parting. These phases, as mentionedearlier, cannot be protected by an inhibitor, thus it has been foundessential to quickly cool castings or wrought material from over 500C toprevent these phases from forming.

Field tests in public water supplies in major cities in the US haveprovided further evidence of the advantages of the present alloys. Acast ASTM B 198, CDA

TABLE IV Corrosion Rate (Millimeters per year) ASTM B 198 ASTM 371 (CDA875) (CDA 697) Non Non- Sitc Inhibited Inhibited Inhibited InhibitedWashington, D.C. 1.6 0.06 0.4 0.06 Cleveland, Ohio 1.1 0.04 0.4 0.]Pasadena, Cal. 04 0.05 0.2 006 Philadelphia, Pa. 0.6 0.1 0.3 0.1

Metallographic examination revealed that in the cast alloy, zeta phasewas heavily attacked in the noninhibited form whereas the inhibitedcounterpart was only superficially affected. In the wrought 697 alloy,both alpha and zeta suffered parting corrosion without an inhibitorpresent but when inhibited, this form of attack virtually disappeared.

I claim:

1. A silicon brass alloy resistant to parting corrosion consistingessentially of about 3-21 percent by weight zinc, an amount of siliconin the range of about 2.5 to about 7 percent, said amounts of zinc andsilicon being sufficient to produce a structure consisting of alpha pluszeta phases in the brass, from about 0.030 percent up to the percentageby weight of solid solubility of one or more elements of the groupconsisting of arsenic, antimony and phosphorus remainder essentiallycopper, said alloy having been rapidly cooled to room temperature from atemperature in the range of 500c to 760C and consisting of alpha pluszeta micro structure.

2. The alloy of claim 1 having about 0.06% by weight arsenic as theelement.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO.

DATED INVENTOR(S) I Column Column Column Column [SEAL] LOUIS P. COSTAStt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

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"500C" should read -500C Signed and Scaled this twenty-third D3)! OfDecember I 975 Arrest:

RUTH C. MASON Arresting Officer C. MARSHALL DANN Commissioner nj'latenlsand Trademarks

1. A SILICON BRASS ALLOY RESISTANT TO PARTING CORROSION CONSISTING ESSENTIALLY OF ABOUT 3-21 PERCENT BY WEIGHT ZINC, AN AMOUNT OF SILICON IN THE RANGE OF ABOUT 2.5 TO ABOUT 7 PERCENT, SAID AMOUNTS OF ZINC AND SILICON BEING SUFFICIENT TO PRODUCE A STRUCTURE CONSISTING OF ALPHA PLUS ZETA PHASES IN THE BRASS, FROM ABOUT 0.030 PERCENT UP TO THE PERCENTAGE BY WEIGHT OF SOLID SOLUBILITY OF ONE MORE ELEMENTS OF THE GROUP CONSISTING OF ARSENIC, ANTIMONY AND PHOSPHORUS REMAINDER ESSENTIALLY COPPER, SAID ALLOY HAVING BEEN RAPIDLY COOLED TO ROOM TEMPERATURE FROM A TEMPERATURE IN THE RANGE OF 500*C TO 760*C AND CONSISTING OF ALPHA PLUS ZETA MICRO STRCTURE.
 2. The alloy of claim 1 having about 0.06% by weight arsenic as the element. 